1. Field of the Invention
The present invention relates to a single crystal pulling apparatus which pulls a silicon single crystal and the like by Czochralski method (hereinafter referred to as CZ method), more particularly to a single crystal pulling apparatus which is suitable for preventing from generation of dislocations in a single crystal due to dusts attached to isolation valves and the apparatus.
2. Description of the Prior Art
As shown in FIG. 8 prior art, a single crystal pulling apparatus 1b to grow a single crystal by the CZ method comprises for the most part a lower chamber 18, an upper chamber 22, and a valve containing portion 24. The lower chamber 18 accommodates a quartz crucible 15 for storing a silicon melt 14, a heater 16 serving as a heating means, and a heat insulating material 17 and the like. The upper chamber 22 accommodates a wire 21 and a driving mechanism (not shown) for pulling the single crystal 19 via a seed crystal 20. The valve containing portion 24, which is disposed between chambers 18 and 22, accommodates an isolation valve 23 which shuts off chambers 18 and 22.
The isolation valve 23 has a shape such that the valve 23 closes and opens an upper opening portion 25 of the lower chamber 18, and comprises a seal means, a cooling means, and a vacuum keeping means and the like. When the grown single crystal 19 is taken out from the apparatus 1b, the grown single crystal 19 is lifted up above the isolation valve 23, and at this situation the upper opening portion 25 of the lower chamber 18 is closed by the isolation valve 23. Keeping the lower chamber 18 at a reduced pressure condition, the upper chamber 22 is restored to a normal pressure, and the single crystal 19 is taken out from the apparatus 1b.
After, the pressure of the upper chamber 22 is reduced, and the isolation valve 23 is opened, pulling of the single crystal 9 is carried out.
However, as the diameter of the single crystal 19 to be grown is larger, the isolation valve 23 also becomes larger. The structure of the valve 23 is apt to be more complicated. On the other hand, a temperature in the single crystal pulling apparatus is high. SiO gas is generated from the melt in the crucible. The SiO gas diffused toward upper direction and deposits on the inner wall of the apparatus so that a film is formed. Exfoliation of this film causes dusts. Furthermore, the SiO gas invades into the valve containing portion 24. A film is also formed on the isolation valve 23 and this film causes dusts. In case the above dusts fall into the crucible 15, the single crystal 19 formes dislocations therein furthermore, heating the isolation valve 23 damages the sealing means, the cooling means, and the vacuum keeping means.
There is a technology disclosed in Japanese Patent Application Laid Open No. 24619/1989 as a prior art to solve the foregoing problems. The single crystal pulling apparatus disclosed in the above literature has a structure that a closing cylinder for closing the entrance of an isolation valve is disposed, at a lowermost portion of an upper chamber, movably up and down along an inner wall of the upper chamber. This prior art prevents the attachments of the dusts to the isolation valve.
As described above, the closing cylinder moves upwardly and downward along the inner wall of the upper chamber so that the dusts originated from exfoliation of a film formed on the inner wall of the upper chamber will fall in a crucible with the motion of the closing cylinder. For this reason, the crystal being formed has dislocations so that inferior goods may be produced. Thus, reduction in productivity and high cost will be brought about.